Discerning Taste

Scientists have for the first time found a protein on the tongue that allows us to taste our food. The finding, reported in the February Nature Neuroscience, means that gourmets can add savory glutamate to their palate of sweet, salty, sour, and bitter.

For all the pleasure it gives us, the sense of taste probably evolved to allow animals to screen food for nutritional value. Sweetness, for instance, often is a giveaway for high-calorie carbohydrates, while minerals usually taste salty. In 1908, a Japanese scientist, working with seaweed broth, isolated a chemical that gives many protein-rich foods a hearty flavor. Calling the taste umami, he found the underlying chemical to be glutamate-- the most abundant amino acid building block of all proteins.

The hunt was on for the proteins on the tongue's surface that latch onto the chemical constituents of taste. Nothing turned up. But at least researchers had a lead on how glutamate may tickle the tongue: In the brain, more than 20 glutamate receptors help transfer signals between neurons. About 4 years ago, neuroscientist Nirupa Chaudhari and her team at the University of Miami School of Medicine found hints of a brain glutamate receptor called mGluR4 in the taste buds of rats. Puzzling, however, was the fact that rat taste buds need about 100 times more glutamate to respond than do brain receptors. To many other scientists, this and other findings suggested that the taste of glutamate may be a blend of the four basic tastes--sweet, salty, sour, and bitter--rather than a unique sensation.

Upon closer analysis, however, Chaudhari's team found that the taste bud cells produce a glutamate receptor lacking about half of the antenna-like structure that, in the brain, protrudes from the cell and binds glutamate. This deficit, the team found, made the taste receptor about 100 times less sensitive to glutamate. "It fits the bill," says Chaudhari.

The finding is "sensational," says Bernd Lindemann, a physiologist at the Saar University in Hamburg, Germany, because it is the first instance in which taste can be seen operating on a molecular level.